Nonaqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, have high energy densities, and are often used as power supplies for mobile phones, laptop computers, and the like. In recent years, use as automotive power supplies for electric vehicles, etc., has also been considered.
Generally, a nonaqueous electrolyte secondary battery is designed as follows:
a positive electrode plate, which is prepared by forming a positive composite layer containing a positive active material on the surface of a positive current collector, and a negative electrode plate, which is prepared by forming a negative composite layer containing a negative active material on the surface of a negative current collector, are placed to face each other via an electrically insulating separator, allowing ions to be transferred between the positive electrode and the negative electrode through a nonaqueous electrolyte having a supporting salt dissolved in a nonaqueous solvent, thereby performing charge-discharge.
Meanwhile, a nonaqueous electrolyte secondary battery is generally controlled so that the voltage will not go beyond a predetermined range. However, when a current is forcibly supplied to the battery for some reason, the battery may turn into an overcharged state, that is, may have a voltage over the energy storage capacity of the battery. In such an overcharged state, the nonaqueous solvent may undergo an oxidation decomposition reaction on the surface of the positive electrode, or lithium metal may be deposited as dendrites on the negative electrode, causing a short circuit. The suppression thereof has been an important issue in nonaqueous electrolyte secondary batteries.
Conventionally, as a measure against the overcharged state of a nonaqueous electrolyte secondary battery, an overcharge inhibitor is added to the nonaqueous electrolyte. As overcharge inhibitors, compounds that undergo oxidation polymerization in an overcharged state to form a high-resistance film form on the surface of an active material, compounds that undergo an oxidation reduction reaction to cause self discharge or internal short-circuiting, compounds that cause gas generation to operate an internal-pressure-operated electricity cutoff valve, and the like are known. For example, Patent Document 1 discloses that aromatic compounds such as toluene, ethylbenzene, cyclohexylbenzene, 4-t-butyl toluene, and biphenyl are usable as overcharge inhibitors.